Place slivers of garlic on the inside of the turkey breast and roll into neat package, tie with kitchen twine. Rub olive oil all over the roast and season with salt and pepper. If turkey breast was purchased tied, make small slits on the outside of the roast and insert the slivered garlic into the slits.
Put the onion, celery, and chopped garlic in roasting pan with 1/2 cup of the stock. Lay seasoned turkey breast roll on top. Lightly tent pan with foil. Add more stock, as necessary, to keep vegetables from burning. Roast for about 45 minutes, until internal temperature registers 160 degrees F.
Remove and let cool in pan. When turkey is room temperature, wrap well in plastic wrap, and refrigerate at least 4 hours or up to 2 days. Discard the onion, celery, and garlic.
To make the sauce: Put the tuna, anchovies, 2 tablespoons of parsley, olive oil, and lemon juice in blender. Using short pulses, puree the mixture. Scrape the sides down frequently and stir to ensure even blending. When smooth, add the mayonnaise, capers, and white pepper and blend. Add the cream, a little at a time until you have desired texture and flavor. Additional white pepper, lemon, or anchovy may be added at this point, if desired. The sauce may be prepared up to 2 days in advance and refrigerated.
To serve, arrange the selection of greens, fennel, and corn on platter. Drizzle lightly with olive oil, season with salt, pepper, and a squeeze of lemon. Slice turkey thinly, about 1/8-inch thick, and arrange on top of the greens. Drizzle with the sauce. Garnish with minced parsley and olives.
For a buffet party presentation, you could surround the Tonnato with marinated roast red peppers and mushrooms.